(I can see what's the issue here: older OSS versions expressed ability to have multiple sound client (using vmix) by splitting them to /dev/dspN where each client uses a different node, while 1050 uses a single /dev/dsp and clones it, so you only see a single engine when vmix is off. /dev/dsp was pure hw earlier, now we need to disable software conversions - if mplayer were to open with O_EXCL than disabling vmix wouldn't have had to be disabled) I wonder if setting vmix0-enable to ON in the mixer, and setting vmix0-channels to Multich will work? Alternatively, does setting vmix0-src to OFF while vmix is enabled work?

Btw, I find these "Unknown mixer extension type" messages to be rather weird since these types do exist in OSSv4's soundcard.h... If you can edit cmd/ossmix/ossmix.c to report which names trigger this, it would be nice (Alternately, it could be a compile issue, but I doubt it).